Mineralogy of siliceous concretions, cretaceous of ionian zone, western Greece: Implication for diagenesis and porosity

• Published in: Marine and petroleum geology 2019-07, Vol.105, p.45-63
• Main Authors: Bourli, Nicolina, Kokkaliari, Maria, Iliopoulos, Ioannis, Pe-Piper, Georgia, Piper, David J.W., Maravelis, Angelos G., Zelilidis, Avraam
• Format: Article
• Language: English
• Subjects: Kastos island; Nodules; NW Peloponnesus; Siliceous beds; X-ray diffraction
• ISSN: 0264-8172; 1873-4073
• DOI: 10.1016/j.marpetgeo.2019.04.011

Siliceous (chert) concretions are a prominent component of Cretaceous limestones in the Ionian zone of western Greece. This study investigates the diagenetic evolution of such cherts from two localities: largely undeformed strata on Kastos Island and highly faulted strata at Araxos. In both areas, lower Cretaceous limestones contain abundant small chert nodules, whereas in the upper Cretaceous nodules are larger but less abundant. Seventy seven powdered samples, from 67 representative field samples, were analyzed by X-ray Powder diffraction (XRPD). The analyzed samples came from both nodular and siliceous beds and showed that quartz is the dominant mineral, with rare to common residual calcite. The presence of moganite and opal-A minerals suggests that quartz recrystallized from original amorphous silica. A few samples from Araxos contain traces of halite and anhydrite. The variability in the calcite content and the presence/absence of halite and anhydrite in the different studied areas are related with the different tectonic activity in these areas, with more intense tectonic activity in NW Peloponnesus than in Kastos Island. Size of nodules is related to variation in primary porosity of the host rocks. Finally, the presence of dolomite was related with the pre-existing underlying dolomites of Early Cretaceous age in Ionian basin, whereas the presence of maghemite is due to the presence of iron minerals in water solutions. •Zoned chert nodules and beds in Cretaceous calcturbidites and landslide deposits.•Moganite and opal-A minerals recrystallized from original amorphous silica.•Size of chert concretions related to porosity of host limestones.•Dolomite in cherts only where underlying dolostone.•Halite and anhydrite in cherts due to Paleogene deformation of Triassic evaporites .